The present invention relates to industrial process field devices which are used to measure or control process variables of an industrial process. More specifically, the present invention relates to an industrial process field device which includes an optical isolator to isolate components within the field device.
An optical isolator, also known as an opto-isolator, optocoupler, or photo coupler, is an arrangement of electrical components that transfers electrical signals between two isolated circuits using electromagnetic radiation including visible light and infrared radiation. This isolation prevents large voltages induced or otherwise present in one of the circuits from being transferred or coupled to the other circuit. Optical isolators cannot supply power between the circuits, but can convey signals between the isolated circuits. As can be appreciated, optical isolators enjoy widespread use in a variety of electronics where isolation is desired.
One particular electronic device that employs optical isolators is known as a field device. Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such field devices have a field-hardened enclosure so that they can be installed in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration and mechanical shock. Field devices also typically operate on relatively low power. For example, some field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
The environment within which the field devices operate can sometimes be highly volatile. Some environments may be so volatile that an errant spark or sufficiently high surface temperature of an electrical component could cause the environment to ignite and generate an explosion. In order to ensure that such situations do not occur, intrinsic safety specifications have been developed. Compliance with an intrinsic safety requirement helps ensure that even under fault conditions, the circuitry or device itself cannot ignite a volatile environment. Optical isolators can be used to isolate circuitry within a field device in order to meet intrinsic safety requirements. Isolated high speed communication between components in a field device typically requires high power optocouplers. However, in many field devices the available power is very limited. Further, optocouplers are typically not configured for use in accordance with intrinsic safety requirements and therefore require a relatively large separation between the transmitter and receiver of the optocoupler. This spacing reduces the speed of the device and requires additional power to span the distance.